This invention is generally related to the manufacture of dimethyl acetamide (DMAc) solvent from the raw materials acetic acid and dimethyl amine and, more particularly, to a process for increasing the conversion of raw materials to dimethyl acetamide by increasing the pressure in the reactor vessel.
Use of dimethyl acetamide as a solvent is expanding since the environmental classification of dimethyl formamide was determined to be carcinogenic. Dimethyl acetamide is used as a solvent for plastics, resins, gums, and electrolytes. It is also used in catalysis, as a paint remover, and as a high purity solvent for crystallization and purification.
DMAc is formed by two reactions starting with dimethylamine (DMA) and acetic acid (HOAc). The first reaction typically occurs in a scrubber column where glacial acetic acid and DMA contact each other in an exothermic reaction to form an intermediate salt, dimethyl ammonium acetate (DMAA). The salt is then broken down to dimethyl acetamide (DMAc) and water with the addition of heat. This reaction is temperature dependent. This second reaction typically occurs in a reactor and a reactor column. DMA is fed to the reactor. Past experience shows that approximately ⅓ of the DMA reacts in the reactor and reactor column, and the remainder passes through a partial condenser as vapor to react in the scrubber. This invention improves conversion so that approximately ⅔ of the DMA reacts in the reactor and reactor column.
Recovered acid is combined with a reactor purge stream and fed to a heels column where the higher boiling monomethyl acetamide (MMAc) is separated from DMAc and HOAc. The concentrated MMAc is removed batch-wise from the heels column sump to a pan dryer. The recovered acid and DMAc are then pumped from the heels column overhead back to the scrubber column where the recovered acid can contact the DMA to form the salt, DMAA.
Attempts to increase conversion in the past have involved different dimethyl amine (DMA) feed locations in the reactor, and different reactor column packing and vapor/liquid distributors. None of these changes affected manufacturing capacity or conversion efficiency. All prior DMAc manufacturing technology uses back pressure on the reactor created by the process equipment and does not make any attempt to control reactor pressure.
Demand for dimethyl acetamide is increasing, and a need exists for lower cost manufacturing processes and equipment.
The present invention concerns a process for manufacturing dimethyl acetamide, which includes the steps of: reacting acetic acid and dimethyl amine to form an intermediate salt, typically dimethyl ammonium acetate; converting the salt into dimethyl acetamide and water in a reactor; maintaining a substantially constant elevated pressure inside the reactor by using an automated pressure control valve; and separating the dimethyl acetamide, for example from unreacted acetic acid, dimethyl amine, and water. Preferably the pressure in the reactor is between about 2 and 10 pounds per square inch gauge.
The acetic acid fed to the process preferably is glacial acetic acid. It is also preferred that the reaction of the acetic acid and dimethyl amine take place at least partially in a scrubber column. It is further preferred that the pressure control valve be controlled by a feedback loop.
In another embodiment of the invention, the process includes the steps of: reacting acetic acid and dimethyl amine to form an intermediate salt; converting the salt into dimethyl acetamide and water in a reactor; maintaining a substantially constant elevated temperature inside the reactor using an automated pressure control valve; and separating the dimethyl acetamide. Preferably in this embodiment the temperature in the reactor is from 163 to 180xc2x0 C.
One specific embodiment of the invention is a process for manufacturing dimethyl acetamide, comprising the steps of: reacting acetic acid and dimethyl amine to form dimethyl ammonium acetate; converting the salt into dimethyl acetamide and water in a reactor; maintaining a substantially constant pressure of from about 2-10 psig and a substantially constant temperature of from about 163-180xc2x0 C. inside the reactor by using an automated pressure control valve; and separating the dimethyl acetamide from water.
The process of the present invention increases the conversion of raw materials into DMAc by increasing the temperature at which the reaction occurs. In one particular embodiment of the process, the higher temperature is achieved by installing a pressure control valve in a dimethyl amine recycle line returning to the scrubber column. This raises back pressure on reactor and reactor column. The higher pressure raises the temperature in reactor vessel. Higher temperature increases the reaction rate and efficiency. The implementation of this invention greatly increases the capacity of existing process equipment without requiring large amounts of capital for additional equipment. Therefore, the present invention allows more economical production of DMAc.